The regulation of organ size is a fundamental mechanism in development. The skin appendage is the best model to study size regulation because of the large number of appendages and the extensive size variation found within one individual organism. Many clinical conditions such as alopecia, hirsutism and even cancer are basically problems of size. The chicken feather is an excellent model because of the large spectrum of graded feather sizes on one individual and the possible variations in special breeds. We hypothesize that the size of feathers is determined through multiple stages of epithelial-mesenchymal interactions, from initiation to growth to resting phases, each modulated by combinatorial positive and negative molecular regulators. We will do molecular mapping of different phases of feather follicles with in situ hybridization and immuno-staining. Preliminary data showed the presence of members of major signaling pathways (e.g. SHH, BMP, Wnt) in different regions of the follicle. We also have identified a novel feather follicle bulge zone that shows long term BrdU label retention that may contain feather stem cells. To search for cellular determinants of feather size, we will engineer chimeric feather follicles made with components from different-sized follicles. To identify molecular determinants, we have developed a feather plucking/regeneration model and can introduce genes to the regenerating feathers using gene therapy technology. Candidate molecules will be chosen by comparative molecular mapping of different-sized follicles, and library subtraction. Preliminary data suggest that the SHH pathway is one of the positive regulators and the BMP pathway is one of the negative regulators for feather size. Alterations of cell proliferation, differentiation (judged by keratin markers) and other signaling molecules will be analyzed to identify crosstalk among molecular pathways. This work will lead to a new understanding of the molecular basis of the size regulation of skin appendages.